The present disclosure relates generally to a method, apparatus, and product, for acquiring a cardiac image from a patient having a paced heart rhythm, or abnormal EKG (such as caused by Left Branch Bundle Block, Atrial fibrillation, or other heart diseases), and particularly to the processing of a gated electrocardiogram signal, from a patient having a paced heart rhythm or abnormal EKG, for improving the quality of the cardiac image.
Patients having cardiac electrical abnormalities, such as from pacemakers, Left Branch Bundle Block, Right Branch Bundle Block, intra ventricular conduction defect, or any other form of conduction defect disease, are often candidates for undergoing cardiac computed tomography (CT) imaging to assess the effectiveness of pacing therapy, particularly in biventricular pacing. Two factors that are taken into consideration in medical imaging are image quality and radiation exposure. To improve image quality, cardiac imaging in most modalities, such as CT, magnetic resonance (MR), nuclear, and others, requires gating of the images to a particular phase of the cardiac cycle. The timing of gating of the cardiac image is usually calculated with respect to the detected R-wave from an electrocardiograph signal from the patient. The duration of a representative cardiac cycle is typically based on the average or median of three or more previous R-to-R intervals, and the phase of the cardiac cycle of interest is typically based on a percent of the representative cardiac cycle duration. However, appreciable variation of paced heart rhythms and abnormal EKG complexes may degrade the image quality and interfere with the means to reduce the radiation dosage using electrocardiogram (ECG, or alternatively EKG) modulation for example.
The presence of pacer (pacemaker) spikes and intrinsic beats in a paced EKG, particularly those that produce variable trigger delays, may lead to poor image quality (misregistration observed as a projection or abrupt offset of an otherwise smooth edge or surface of the reconstructed cardiac image) due to the reconstruction of cardiac images from incorrect phases of the cardiac cycle. As used herein, trigger delays refers to a delayed trigger (positive delay) or an early trigger (negative delay). Misregistration may occur in a variety of imaging modalities, including helical CT, electron beam CT, MR, Nuclear/PET (positron emission tomography), and other modalities that use gating for reconstructing images at selected phases of the cardiac cycle. Trigger delays as small as about 15 milliseconds have been found to result in poor image quality in some instances. When substantial trigger delays occur due to pacing, images from incorrect phases of the cardiac cycle are combined with images from correct phases, resulting in misregistration. As a result, R-wave delay, relative to the duration of the representative cardiac cycle, is a contributing factor to misregistration and poor image quality in patients having irregular heartbeats. As used herein, the term irregular heartbeat includes paced heart rhythm and abnormal EKG.
Accordingly, there is a need in the art for an apparatus and method for cardiac imaging of a patient having an irregular heartbeat that overcomes these drawbacks.